Dynamical analysis of seasonal and interannual variability in the equatorial Pacific

Time series measurements from the Tropical Atmosphere Ocean array during th e 1980s and 1990s are used to analyze the dynamics of the long-term mean, t he seasonal cycle, and interannual variability in the upper 250 m of the eq uatorial Pacific. The analysis is based on a diagnosis of the zonal momentu m equation using surface winds, ocean temperatures, and currents between 16 5 degrees E and 110 degrees W along the equator. In the mean, the balance i s mainly between zonal wind stress and depth-integrated zonal pressure grad ient, in good agreement with linear Sverdrup theory. For the seasonal cycle , the balance is mainly between wind stress, pressure gradient, and local a cceleration. with differences between the stress and pressure gradient lead ing to accelerations and decelerations of the South Equatorial Current and Equatorial Undercurrent. On interannual timescales, local acceleration is m uch smaller than either zonal wind stress or zonal pressure gradient, indic ating that to zeroth order, variations in the equatorial Pacific are in qua si-equilibrium with the wind forcing. Consistent with this quasi-equilibriu m balance, Equatorial Undercurrent transports in the thermocline vary nearl y in phase with the strength of the zonal pressure gradient and the easterl y trade winds. Nonlinear effects are significant on all timescales, though of secondary importance in the depth-integrated zonal momentum balance.